1. Field of the Invention
The present invention relates to video image signal processing, and more particularly, to an adaptive contrast and brightness enhancement method and apparatus for preserving colors of an image being displayed on a display device.
2. Description of the Related Art
It is known in video image processing that in an environment where the dynamic ranges of video images output to a display device are restricted, the levels of incoming video images are controlled or transformed by a non-linear function to enhance the contrast of these video images. In this case, the non-linear function is already fixed, or made variable based on the histogram of input video images.
Where the levels of incoming video images are simply transformed by the non-linear function, an undesirable white element is generated and saturation is reduced. The contrast at a specific level of the video images within a limited range may improve, but, to improve contrast while maintaining color hues, an approach to adjusting gain depending on the brightness direction components of RGB vectors of incoming video images has been proposed. This approach involves increasing the expansion gain of a signal, which is close to an achromatic color due to its large brightness direction component while decreasing the expansion gain of a signal having a high saturation and a large color-difference component.
This technique makes it possible to increase the overall brightness while maintaining the saturation of colors. However, it has a drawback in that adjusting gain only according to the colors of an image makes selective expansion depending on the luminance level of a video image impossible and saturation occurs due to expansion of the dynamic range.
A need therefore exists for a system and method for selectively increasing contrast depending on the level of an incoming video signal while preserving color.